In a conventional wireless LAN system making use of radio as a transmission medium, for example, in a radio communication system disclosed in Japanese Patent Application laid open No. HEI9-162798 (FIGS. 14 and 20), power-saving operation in a radio terminal unit is carried out by intermittently receiving beacons from a radio base station. Besides, it is required to receive at least a beacon having a delivery traffic indication map (DTIM) in order to obtain a multicast/broadcast packet.
That is, when a radio terminal unit enters power-saving mode, the radio terminal unit obtains a beacon transmitted from a radio base station. Having extracting respective information elements included in the beacon, the radio terminal unit carries out intermittent receiving operation based on beacon interval information included within the beacon and an interval between the transmission of beacons each having a delivery traffic indication map (hereinafter referred to as “DTIM”).
When operating in power-saving mode, the radio terminal unit informs the radio base station of its operation using a frame control field. The radio base station buffers packets addressed to the radio terminal unit operating in power-saving mode in a memory within the radio base station, and informs the radio terminal unit that the packets have been buffered by a traffic indication map (hereinafter referred to as “TIM”) included in a beacon.
As just described, a radio terminal unit operating in power-saving mode receives beacons intermittently from a radio base station. After receiving each beacon, the radio terminal unit extracts information elements. Having recognized from the TIM that the packets sent to the radio terminal unit itself had been buffered, the radio terminal unit sends the radio base station a control packet (hereinafter referred to as “PS-Poll”) requesting to deliver the buffered packets. Thus, the radio terminal unit receives its packets buffered by the radio base station.
Additionally, a multicast/broadcast packet is sent to the radio terminal unit subsequent to a beacon with the DTIM. The radio terminal unit receives at least a beacon with the DTIM in order to obtain the multicast/broadcast packet.
A power saving to the radio terminal unit can be achieved by lengthening the interval between the receiving of beacons from the radio base station if there is no traffic. However, in the case where there are packets for the radio terminal unit when the receiving interval has been lengthened, the radio terminal unit can be late in obtaining the TIM, which causes delay in receiving the packets.
Besides, the radio base station buffers packets addressed to the radio terminal unit operating in power-saving mode in its memory. Consequently, when the receiving interval is lengthened on the radio terminal unit operating in power-saving mode, delivery of packets to the radio terminal unit is delayed. Thus, the radio base station has to retain the packets in the memory for a long period.
In the real-time communication of voice, moving images, etc., if the radio terminal unit which is in power-saving mode repeats transmission and reception at long receiving intervals, packets addressed to the radio terminal unit are once buffered by the radio base station. The buffered packets are delivered to the radio terminal unit in the next receiving period, and, therefore, delay occurs in packet delivery. Especially, in the real-time communication of voice, moving images, etc., delay occurs in receiving packets on the radio terminal unit, which may cause a problem in the reproducibility of data.
In addition, the radio terminal unit in power-saving mode carries out the intermittent receiving operation in timing with transmission of beacons each having the DTIM from the radio base station. That is, the radio terminal unit cannot determine the timing of the intermittent receiving operation.
Moreover, in the case where a plurality of radio terminal units are connected to the same radio base station and operate in power-saving mode, the respective radio terminal units have to perform the intermittent receiving operation based on the same intermittent receiving timing, that is, the timing of transmission of beacons each having the DTIM from the radio base station.
Further, the multicast/broadcast packet is sent to the radio terminal unit subsequent to a beacon with the DTIM. Therefore, the radio terminal unit, which operates in power-saving mode at long receiving intervals without reference to the beacon having the DTIM, may not be able to receive the multicast/broadcast packet. For example, in the case where a physical address resolution protocol message (ARP) is issued in a network for inquiring the physical address of the radio terminal unit operating in power-saving mode, delay occurs in delivery to the radio terminal unit. As a result, the network is congested with the traffic caused by retransmission.
Still further, in the wireless LAN communication, the CSMA/CA (Carrier Sense Multiple Access protocol with Collision Avoidance) procedure is performed in order to avoid a collision during data transmission. Regardless of communicating application, the same DIFS (Distributed Inter Frame Space) is applied to every PS-Poll which the radio terminal unit transmits for requesting a radio base station to send packets buffered therein when the radio terminal unit is operating in power-saving mode. Therefore, it is impossible to minimize delays and to give transmission right preferentially to real-time communication such as voice communication.
Moreover, the back off algorithm is used when data are to be transmitted. That is, data are actually transmitted when random wait time has passed after the transmission right was given. Consequently, it is required to wait the random time regardless of the contents of send data, which makes it impossible to minimize delays in real-time communication such as voice communication.
In order to solve the above-mentioned problems, the inventors have proposed “a radio terminal and a radio communication system using the same” in Japanese Patent Application No. 2002-291063. The radio communication system comprises a radio base station and radio terminal units each being connected with a LAN or a WAN.
FIG. 1 is a timing chart illustrating the operation of the conventional radio communication system for transmission and reception. In the following, the operation of the radio communication system will be schematically described referring to FIG. 1.
Referring to FIG. 1, the radio terminal units 610, 620 and 630 belong to a radio base station 600. The radio base station 600 once buffers packets to the radio terminal units that carry out the intermittent receiving operation, and sends the packets to the respective radio terminal units on receipt of the PS-Poll (a control packet for requesting a radio base station to deliver buffered packets).
The radio terminal units 610, 620 and 630 perform the intermittent receiving operation at different intervals, respectively, in synchronism with multiples of beacons transmitted by the radio base station 600 at a regular time interval. The intermittent receiving interval for each of the radio terminal units 610, 620 and 630 is determined according to the operation mode of one or more communication applications running on each terminal unit. In the example of FIG. 1, a “web browser” application is active on the radio terminal unit 610, a “chat” application is active on the radio terminal unit 620, and a “VoIP” (Voice over Internet Protocol) application is active on the radio terminal unit 630. The intermittent receiving interval is selected from divisors of the interval between the transmission of DTIM beacons (beacons each containing the DTIM) within the range of the beacon interval (an interval between the transmission of respective beacons) to the DTIM beacon interval (an interval between the transmission of respective DTIM beacons). In other words, the shortest intermittent receiving interval is the beacon interval, and the longest is the DTIM beacon interval. Accordingly, each radio terminal unit can change the intermittent receiving interval depending on the operation mode of the application running on it. Thereby power-saving control on the radio terminal unit can be executed accurately.
Besides, the radio terminal units 610, 620 and 630 can maintain the priorities of the PS-Polls transmitted to the radio base station 600. Such priorities are set based on the operation mode of the application running on the radio terminal units 610, 620 and 630. Consequently, packets of real-time communication such as voice packets can be preferentially transmitted. Thus, it is possible to reduce delays and improve voice quality.
With the conventional technique, each radio terminal unit transmits the PS-Poll to the radio base station triggered by the receipt of a beacon so as to receive packets addressed to it. In the case where a plurality of radio terminal units are connected to one radio base station and a communication application that is required to perform real-time processing, such as a VoIP application, is running on the respective terminal units, the plural terminal units send the PS-Polls to the radio base station, respectively, each time they receive beacons. Consequently, it is highly likely that the period of wait time from when the transmission right was given to data to when the data are actually transmitted is prolonged. As a result, delays are increased, which affects voice quality.